Projection systems have been used for years to project motion pictures and still photographs onto display screens. More recently, presentations using multimedia projection systems have become popular for purposes such as sales demonstrations, business meetings, and classroom sessions. In a common mode of operation, multimedia projection systems receive analog video signals from a personal computer (PC). The video signals represent still, partial-, or full-motion display images of the type rendered by the PC. The analog video signals are converted into digital video signals to control a digitally-driven light valve, such as a reflective light valve or a transmissive liquid crystal light valve (LCD), which form display images.
A popular type of multimedia projection system is a projector that incorporates a light source and optical path components upstream and downstream of the light valve to project the display images onto a display screen. Examples such projectors include those sold under the trademark LITEPRO.RTM. by In Focus Systems, Inc. of Wilsonville, Oreg., the assignee of the present application.
It is increasingly necessary that projectors be capable of projecting electronic images in a variety of formats. Two of the most common formats are the 3.times.4 standard screen format and the 16.times.9 "letterbox" format. The letterbox format is utilized in some laser-disc based videos and is proposed for use in high-definition television (HDTV).
While conventional projectors are capable of handling both standard and letterbox formats, they do so with a penalty in optical efficiency. For example, the optical systems of conventional projectors are usually designed for standard format projection, and letterbox format projection is accomplished by "blanking" pixels in upper and lower margin portions of the light valve. In the case of a transmissive light valve, the blanked margins block light while the active central portion of the light valve modulates and passes light, producing a letterbox-sized display field. In the case of a reflective light valve, the blanked margins block light by reflecting it away from the projection path, and the active central portion of the light valve modulates and reflects light along the projection path, producing a letterbox-sized display field.
In both cases, the light blocked by the upper and lower blanked margin portions is wasted, thereby compromising projector brightness in the letterbox mode. Such decrease in projector brightness is a serious disadvantage. For instance, even in standard display format without letterbox blanking, the optical efficiency of many projectors is such that the projectors are only marginally bright enough to produce good image quality in a well-lit room. Thus, the additional loss of light attending letterbox blanking is undesirable.
Thus, in light of these disadvantages, it is a principle object of the present invention to provide a projector capable of standard and letterbox format projection, without loss of brightness in the letterbox display format.
It is another object of the present invention to provide a projector with a simple system for converting between standard and letterbox formats.
In accordance with a preferred embodiment of the present invention an image projector is provided with a light source and a light valve positioned downstream of the light source. An optical path is defined between the light source and the light valve. To accommodate various image display formats, a plurality of optical integrators is positioned between the light source and the light valve. Each of the plurality of optical integrators is movable laterally of the optical path and is positionable in the optical path. Light is transmitted from the light source to the light valve through the particular optical integrator positioned in the optical path. Each optical integrator is constructed to produce a unique display format for light transmitted therethrough to the light valve.
Accordingly, in one preferred embodiment of the present invention a first and a second optical integrators are provided to accommodate both standard and letterbox viewing formats without waste of light.